1. Field of the Invention
The present invention relates to battery selection circuits for dual batteries used in battery powered electronic devices.
2. Description of the Related Art
Recently, portable or notebook computers have become a popular alternative to traditional desktop computers. These portable computers are typically powered by rechargeable batteries or alternatively by a power adapter which can be connected to an AC outlet. Often, it is desirable to operate the portable computers for lengthy periods of time in environments where AC power is unavailable. An earlier power supply system used in the portable computer includes a rechargeable battery and an AC adapter connected to a power supply circuit of a notebook computer, for example. The power supply circuit includes a reverse current blocking circuit having diodes and a DC-DC converter for receiving DC output voltages from the battery and the AC adapter, and for producing required DC output voltages to be supplied to electronic circuits.
In such a portable computer, the battery is usually formed into a pack including a plurality of battery cells and can be housed in a detachable way. Furthermore, only one battery pack is accommodated for the sake of mobility and convenience. Although the battery pack is composed of a longer-life battery, e.g., Nickel metal hydride (NiMH) or Lithium ion (Li-ion) electrolytes, the limited battery life can be a severe constraint on the effective portable use of a device.
One known way to extend battery life is to carry extra charged batteries. When AC power is not available, and when the battery expires, it can be replaced with another freshly charged one. However, this method can be inconvenient in swapping the batteries, and when the battery is changed, it may require powering off the device which causes a suspension of works proceeding in the computer.
In order to solve this problem, an effort has been made to install multiple battery packs in the portable computer to extend battery life. For example, an extra charged battery can be inserted into the portable computer so that 3.5" floppy disk drive is replaced by the extra battery. This method is based on the fact that the floppy disk drive is hardly used during the execution of word processing programs, for example. Of course, the package of the extra battery must have the same footprint as the floppy disk drive. Thus, a floppy disk drive receiving space of the portable computer can be used by an extra battery pack to increase total battery capacity.
However, in this case, problems arise when the characteristics of two battery packs are different. For example, differences in number of cells and voltages per cell between two battery packs results in a difference of output voltage level. Therefore, the discharging of two batteries is performed at the higher voltage level, that will be substantially ineffective for extending battery life. Further, the battery of a higher voltage level is liable to be discharged excessively and thus this will affect battery life by itself.
The patent to Eguchi et al, U.S. Pat. No. 5,547,775 entitled Circuit For Preventing overcharge And Overdischarge Of Secondary Batteries, discloses a circuit for preventing overcharge and overdischarge of two secondary batteries.
The patent to Stockstad, U.S. Pat. No. 5,646,503 entitled Method For Balancing Power Sources And Structure Therefor, discloses a method of balancing the discharge from two battery cells.
The following patents each disclose features in common with the present invention but are not as pertinent as the two patents discussed in detail above: U.S. Patent No. The Staarman et al, U.S. Pat. No. 5,179,337 entitled Over-Discharge Protection For Rechargeable Batteries, the Morita et al patent, U.S. Pat. No. 5,652,499 entitled Power Supply Apparatus To Provide Power To Electrical Equipment From A Plurality Of Battery Packs And Its Method Of Power Supply, the Tamai patent, U.S. Pat. No. 5,477,124 entitled Circuit To Prevent Excessive Rechargeable Battery Discharge, the Willis patent, U.S. Pat. No. 5,598,041 entitled Efficient Fault Tolerant Switching Circuit For Redundant D.C. Power Supplies, the Allen et al. patent, U.S. Pat. No. 5,477,123 to Allen et al, entitled Connection Multiple Batteries To Battery Powered Devices, the Nanno et al. patent, U.S. Pat. No. 5,553,294 entitled Portable Computer Powered By Rechargeable Batteries, and the Allen et al. patent, U.S. Pat. No. 5,621,301 entitled Connecting Multiple Batteries To Battery Powered Devices.